Removal of Arsenic from Drinking and Process Water

ABSTRACT

A method of removing arsenic and heavy metals from water using metal salt hydroxide-gels is provided. The arsenic present in water is adsorbed onto the hydroxide-gels which can effectively be filtered through a diatomaceous earth (DE) filtration bed. The combination of DE mixed hydroxide-gels is also effective in removing arsenic from water and heavy metals from water.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/510,526, filed Oct. 7, 2004, which is the National Stage ofInternational Application No. PCT/US2003/011010, filed Apr. 10, 2003,which takes priority from U.S. Provisional Application Ser. No.60/371,773, filed Apr. 10, 2002, hereby incorporated by reference to theextent not inconsistent with the disclosure herewith.

BACKGROUND OF THE INVENTION

Arsenic is a naturally occurring element in the earth's crust and can befound in many natural ecosystems. Mining of ores containing arsenicreleases arsenic into the soil. Burning of arsenic containing fossilfuels, volcanic eruptions and weathering processes also introducesubstantial amounts of arsenic into the environment. Various industrialactivities such as smelting, petroleum refining, pesticide and herbicidemanufacturing, glass and ceramic production generate arsenic containingwastewater. The presence of arsenic in natural waters may originate fromgeochemical reactions, industrial waste discharges or agricultural useof pesticides containing arsenic (Gupta and Chen, 1978). Most of thegold and silver ores are closely associated with arsenic compounds.Arsenic is present in the gold cyanidation process. Arsenic is presentin gold extraction processes, which utilize roasting, pressureautoclaving and other oxidation pathways.

The major issue in studies related to arsenic in the environment hasinvolved its deleterious effects on the environment. Arsenic is mobilewithin the environment and may circulate many times in various formsthrough the atmosphere, water and soil before finally entering tosediments (Ahmann et al., 1997).

Hyperpigmentation, skin cancer, liver cancer, circulatory disorders, andother ailments have been attributed to the presence of arsenic in water(National Academy of Sciences, 1977; National Research Council, 1999).The United States Environmental Protection Agency (USEPA) has identifiedarsenic as a group A “known” carcinogen. This classification is based onsufficient evidence of carcinogenicity from human data involvingoccupational and drinking water exposures. Arsenic presents a potentialhealth problem due to its toxicity. In response to these healthconcerns, the USEPA, in January 2001, promulgated the new Arsenic rulethat lowered the maximum contaminant level (MCL) in drinking water to 10μg/L (10 ppb) for both community and non-transient, non-community watersystems. Previously, the Safe Drinking Water Act (SDWA) had a minimumstandard of 50 ppb. USEPA lowered the standard based on recommendationsby the National Research Council (1999) which reviewed scientificstudies on health effects of arsenic on human populations. At present,approximately 4000 drinking water systems in the U.S. will need tocomply with the new standard by January 2006, and many of these systemsare located in rural areas of the West. Conventional water treatmentsystems will cost the nation between $180 and $725 million to meet the10 ppb standard set by EPA.

Arsenic occurs in inorganic form in aquatic environments, resulting fromthe dissolution of solid phases such as arsenolite (As₂O₃), arsenicanhydride (As₂O₅) and realgar (AsS₂). The chemistry of arsenic inaquatic systems is complex because the element can be stable in fourmajor oxidation states (+5, +3, 0 and −3) under different redoxconditions. In natural waters arsenic is found as an anion with acidcharacteristics in only the As(III) and As(V) oxidation states. Inoxygenated waters, the oxyanions of arsenic exist in four differentarsenate species as H₃AsO₄, H₂AsO₄ ⁻, H₃AsO₄ ²⁻ and AsO₄ ³⁻ in the pHranges of <2, 3-6, 8-10 and >12, respectively. Arsenite is more likelyto be found in oxygen free (anaerobic) groundwater, while arsenate ismore common in aerobic surface water. Arsenite ion is oxidized toarsenate in the presence of oxygen, chlorine or potassium permanganate.Therefore under neutral conditions and acidic conditions, As(III) existsas a neutral species and cannot be adsorbed by an adsorbent based onionic interaction alone. The chemistry of arsenic is more fullydescribed in U.S. Pat. No. 6,197,201.

Several methods for reducing arsenic concentrations to acceptable levelshave been studied and are being used currently. These methods includecoagulation and precipitation using ferric chloride and sulfate, ionexchange, reverse osmosis and adsorption using activated carbon andalumina. These methods are effective to a certain extent. However, thesemethods are considerably more expensive and generally narrower inapplication than is desired for the treatment of large volumes of water.

The use of ferric chloride, hydrated lime, sodium sulfate and alum tocoagulate water containing arsenic has been described (Harper andKingham, 1992). These methods require multiple treatments of water withcoagulation chemicals and large amounts of chemicals relative to theamount of arsenic present to obtain the desired reduction in arsenicconcentration. In addition, the methods produce sludge that requiresdewatering or solidification and eventually landfill storage ashazardous waste. Also, the ferric chloride process requires pH of lessthan 6.5 (Merrill et al., 1987).

A method of precipitating arsenite and arsenate ions from aqueoussolutions using yttrium carbonate at alkaline pH has also been described(Wasay et al., 1996). This method requires strict control of pH toachieve removal sufficient to comply with environmental standards. Inaddition, the effective pH range was found to depend on which arsenicspecies was desired to be precipitated.

U.S. Pat. No. 3,956,118 discloses a process for removing phosphate ionsfrom waste waters using a rare earth salt. However, the disclosedprocess is limited to removal of phosphates.

Recently new adsorbents, lanthanum oxide and lanthanum-alumina oxide,have been used for removing arsenate and arsenite species from solution(U.S. Pat. No. 5,603,838,1995). This patent discloses that lanthanumoxide alone or in conjunction with alumina solids and other oxides canremove arsenic to low levels (<50 ppb). Also, the adsorption kineticswere found to be extremely fast compared to other adsorbents such asalumina (Davis and Misra, 1997; Misra and Adutwum, 2000; Misra et al.,1997; Rawat and Misra, 1998).

A novel precipitation process developed by Misra et al. (U.S. Pat. No.6,197,201) uses lanthanum chloride and optionally other salts toselectively coprecipitate arsenite and arsenate from process water(Misra et al., 2000; Nanor, Misra, and Chen, 1999; Nanor and Misra,1998).

General drawbacks of the processes discussed above include inefficientremoval of arsenic to an acceptably low level for drinking water anddischarge into ground water, the problem of filtration of precipitatedsludge and fouling of resins and membranes. In addition, once thearsenic species are removed, the solid materials formed must be disposedof. The solid materials formed from the processes above are alsosusceptible to leaching of the metals at a future time.

There is a need for a highly selective and inexpensive reagent, whicheffectively precipitates and stabilizes arsenate and arsenite speciesover a wide pH range from solutions or solids/liquid mixtures. There isalso a need for an inexpensive, suitable media for the filtration ofadsorbed arsenic species.

BRIEF SUMMARY OF THE INVENTION

This invention is in the field of removal of toxic metals from aqueoussolutions and stabilizing toxic metals, specifically removal of arsenicand heavy metals from aqueous solutions and drinking water.

It was discovered that lanthanum hydroxide is a powerful reagent toprecipitate heavy metals and arsenic as arsenite and arsenate ions, forexample, from aqueous solution at various pHs. The lanthanum hydroxidecan be pure or can be mixed with other elements of the lanthanideseries. Lanthanum hydroxide can also be used in combination with othermetals such as ferric hydroxide, magnesium hydroxide, sodium hydroxideand/or aluminum hydroxide. Lanthanum hydroxide optionally combined withother metal hydroxides are referred to as “metal salt hydroxide-gels”. Apreferred embodiment is the combination of iron chloride and lanthanumchloride at various ratios which form mixed metal salt hydroxide-gels atan appropriate pH which are used to remove arsenic from water, asdescribed herein. When a small amount of lanthanum chloride is added tosolutions containing arsenite and arsenate in the presence of ferricions, the ions substantially precipitate from the solution. Theresultant precipitant is extremely stable and easy to filter. Theprecipitant may be filtered through a bed of diatomaceous earth (DEfilter bed or DE bed), for example. Alternatively, a pre-coat DE bed maybe used, where the DE is coated on a filter screen or septum and thearsenic-containing solution is passed through the pre-coat DE bed. Thefilter screen or septum is a water-compatible material. Some examples ofwater-compatible materials are polyethylene, polypropylene or stainlesssteel, or other suitable material as known to one of ordinary skill inthe art. DE coated with metal salt hydroxide-gels can be used tocoagulate arsenic which can further be filtered through a second DE bedor pre-coat DE bed, if necessary, to reduce the concentration of arsenicto a desired level. Generally, the DE filter bed is made by filteringwater conditioned with DE through a screen. The thickness of the DE bedranges from ½ inch to several inches thick, as known by one of ordinaryskill in the art.

The metal salt hydroxide-gels can be formed prior to contacting withwater to be treated by combining non-hydroxide metal salts at anappropriate pH, forming metal salt hydroxide-gels. These metal salthydroxide-gels are then contacted with the water to be treated.Alternatively, the metal salt hydroxide-gels can be formed uponcontacting the water to be treated with the metal salts, by suitable pHadjustment.

Specifically, provided is a method of removing arsenic fromarsenic-containing water comprising: contacting said water with aprecipitating composition comprising a metal salt hydroxide-gel; andseparating said water from said precipitating composition. Preferablysaid separating is carried out using a filter. The filter is preferablya DE filter bed or a pre-coat DE filter bed.

Also provided is a method of removing arsenic from arsenic-containingwater comprising: coating DE with one or more metal salt hydroxide-gelsto form DE pre-coated hydroxide-gels; and contacting saidarsenic-containing water with said DE pre-coated hydroxide-gels.

Also provided are materials comprising diatomaceous earth coated withmetal salt hydroxide-gels. These materials are made by mixing metalsalts with DE. The hydroxides can be formed by pH adjustment upon mixingor the materials can be stored and the pH adjusted to form hydroxideswhen desired. These materials are further described in Examples Ithrough L.

Also provided are methods of removing heavy metals from heavymetal-containing solutions. The same methods, metal salt hydroxide-gels,and filters described herein for removing arsenic are used to removeheavy metals. Preferred heavy metals include copper, lead and chromium.

Preferably, the metal salt hydroxide-gel is formed from one or moremembers of the group consisting of: lanthanum chloride, lanthanumnitrate, lanthanum carbonate, other rare earth salts, ferric chloride,ferric sulfate, magnesium chloride, magnesium nitrate, magnesiumcarbonate, aluminum chloride, aluminum nitrate, aluminum sulfate, andsodium aluminate. Other salts may be used, as known in the art.Hydroxide salts, such as La(OH)₃ may also be used to form the metal salthydroxide-gel, without precipitating from another metal salt, such aschloride.

The methods of the invention are carried out at a pH which is sufficientto allow the desired reactions to occur, preferably between about 2 toabout 14, more preferably between about 4 to about 10. The metal salthydroxide-gel is contacted with the arsenic- or heavy metal-containingwater for a suitable time to allow the desired level of reduction tooccur, typically between about 1 minute to about 30 minutes. The pH andtime required to allow the desired level of arsenic reduction to occuris easily determined by the methods described herein. The methods of theinvention remove arsenic from arsenic-containing water having a varietyof concentrations of arsenic, preferably about 10 ppb to several ppm.The arsenic-containing water may contain one or more species of arsenic,such as arsenite and arsenate. The methods of the invention may be usedto treat any water, including raw water, well water, drinking water(chlorinated or not), and process water. The methods of the inventionremove heavy metals from heavy metal-containing water having a varietyof concentrations of heavy metals, preferably from about 10 ppb toseveral ppm.

Preferably, the metal salt hydroxide-gel comprises lanthanum and iron,preferably at weight ratios of between about 1:1 and 1:10 and allintermediate values and ranges therein.

In the methods of the invention, the filter is preferably selected fromthe group consisting of: diatomaceous earth (DE), cellulose, andperlite, but any suitable material may be used, as known in the art. Thearsenic-containing water is contacted with the metal salt hydroxide-gelby any means known in the art, including mechanical mixing, ultrasonicmixing, mixing in-line using an atomizer, mixing in-line using venturiand using metering pumps. The methods of the invention may furthercomprise separating the arsenic-removed water from thearsenic-containing hydroxide-gels. This separating may be accomplishedby any means known in the art, including settling, DE-assisted settling,flotation of hydroxide-gels or DE-assisted centrifuge. Thearsenic-containing sludge, consisting of hydroxide-gel and DE, isstable. The arsenic-containing sludge prepared by the methods describedherein passes the Toxicity Characteristics Leaching Procedure (TCLP)test.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a schematic of one embodiment of the invention.

FIG. 2 shows arsenic removal as a function of hydroxide-gel addition.

FIG. 3 shows arsenic removal from Fernley well water using the ex-situmethod.

FIG. 4 shows arsenic removal from Fernley well water using the in-situmethod.

FIG. 5 shows arsenic removal from synthetic arsenite solution using theex-situ method.

FIG. 6 shows arsenic removal from synthetic arsenite solution using thein-situ method.

FIG. 7 shows a flow diagram of one embodiment of the ex-situ process.

FIG. 8 shows a flow diagram of one embodiment of the in-situ process.

FIG. 9 shows arsenic removal from Fernley ground water.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, “removing” means the concentration is reduced to adesired level. For example, “removing arsenic from arsenic-containingwater” means reducing the concentration of arsenic, preferably in theform of arsenite and arsenate, in arsenic-containing water to a desiredlevel, preferably to a concentration below 10 ppb. “Arsenic-containingwater” may contain other elements other than arsenic. “Aqueous solution”refers to a solution in which water is the dissolving medium or solvent.A “precipitating composition” refers to any of the agents describedherein that cause precipitation or stabilization of the ions ofinterest. Preferred precipitating compositions are metal salthydroxide-gels. A “solid” material refers to the resultant materialformed from the contacting of the precipitating composition with theaqueous solution. Solid materials formed include amorphous materials andcrystalline materials or mixtures. The pH of the aqueous solution isadjusted by any means known by one skilled in the art, includingaddition of calcium hydroxide and sodium hydroxide to raise the pH oracid to lower the pH. A “concentration effective for removing at leastone ion” refers to the concentration of precipitating compositionrequired to remove a measurable amount of the selected ion. “Lanthanumchloride” refers to both pure and impure lanthanum chloride. Impurelanthanum chloride can contain various elements of the lanthanide seriesin addition to lanthanum. The lanthanide series of elements includes theelements lanthanum, cerium, praseodymium, neodymium, etc, as known inthe art. Ferric hydroxides can be prepared from ferric chloride, ferricsulfate, and ferric nitrate, as known in the art. Other metal hydroxidescan be prepared from other metal salts by mixing with lime, caustic, andmagnesium oxide, as known in the art. As used herein, “contacting” meansthe arsenic- or heavy metal-containing water and the precipitatingcomposition are placed together so that the desired reaction occurs tothe desired extent. As used herein, a “metal salt hydroxide-gel” is ahydroxide formed from a metal salt, which can be used in the methods ofthe invention to remove arsenic from arsenic-containing water or heavymetals from heavy metal-containing water.

The processes of this invention can achieve the removal of more than 99%of arsenic species in an aqueous solution. Concentrations of less than 5parts per billion arsenic are achievable. The processes of thisinvention can achieve the removal of more than 99% of at least one heavymetal species in an aqueous solution. Concentrations of less than 5parts per billion heavy metal are achievable.

In the methods of the invention, solutions containing arsenic ions arecontacted with a precipitating composition. For the removal orstabilization of arsenic ions, such precipitating compositions comprisefrom about 10-5000 moles of lanthanum or other elements of thelanthanide series for every mole of arsenic ions in solution. Thecompositions can comprise ferric ions in concentrations from about 1mole to about 500 moles ferric ions or other metal other than lanthanideor other elements of the lanthanide series for every mole of arsenicion. The same concentrations are useful to remove heavy metals fromsolution.

The solution should remain in contact with the precipitating compositionfor a period of time sufficient to cause removal of arsenic ions to thedesired concentration ranges. Typically about 5-30 minutes is sufficientto cause precipitation to the desired concentration range and/orstabilization of the arsenic when the concentration of arsenic insolution is between 12-100,000 ppb.

The process of this invention can be used to remove arsenic fromsolutions that contain one or more species of arsenic.

A preferred process of this invention involves adjusting the pH of thesolution to between 6 and about 10, adding 1000 moles of lanthanum forevery mole of arsenic ions present in solution, and adding about 5 toabout 6 moles of ferric ions for every mole of arsenic ions present insolution. The arsenic is stabilized by the metal salt hydroxide-gelsformed. The solution is then typically separated from the solids,preferably through a filter.

Suitable precipitating composition levels depend on the desired level ofarsenic or heavy-metal removal. Generally, a level of between 10 mgprecipitating composition per liter to be treated and 1000 mg/L areuseful, and all individual values and ranges therein.

DE filter aids are generally used in two different modes: as a precoat,wherein the DE acts as the solid/liquid separation surface barrier(albeit with some depth), and as body feed, wherein the DE acts tomaintain a permeable cake structure and create a de-facto depthfiltration. These two modes are used both in combination andindividually, depending on the type and degree of difficulty of thefiltration.

For precoat applications, the DE must be able to form a barrier tightenough to provide the desired clarity of filtrate, while at the sametime be as open as possible to provide maximum filtrate throughput. Whenused as a body feed or ad-mix, the filter aid must be just tight enoughto prevent migration of trapped solids through the forming cake. Theseaspects of filtering are known to one of ordinary skill in the art.

The DE precoat is formed by passing a dilute suspension (generally lessthan 5% by weight) of DE in process water over a septum or screen. Theseptum or screen serves as a support structure for the formation of theDE precoat. As the slurry passes through the septum or screen, the DEsolids are trapped on its surface and a precoat is formed. The precoatthickness can be as little as 1.5 mm and still provide adequatesolid/liquid separation during subsequent filtration. The driving forceduring formation of the precoat can be either negative (vacuum) orpositive pressure. The openings in the septum or screen should be justsmall enough to trap the DE particles, but can be highly variable insize and geometry. The septum or screen can be constructed of anymaterials compatible with water, including stainless steel,polypropylene, polyethylene, and other materials known in the art.

The invention is further illustrated by the following non-limitingexamples.

In the examples below, bulk synthetic solutions were prepared forarsenic. All bulk solutions were made using deionized water. Thelanthanum chloride used can be pure lanthanum chloride or lanthanumcarbonate treated with hydrochloric acid. Ferric chloride can beobtained from a variety of commercial sources. The schematic of apreferred embodiment is given in FIG. 1.

EXAMPLE A Removal of Arsenic from Synthetic Solutions

Arsenic removal from synthetic solutions was conducted using differentprecipitating chemicals. Initial arsenic concentration in the solutionwas maintained at 100 ppb, and pH was adjusted to 6-8.5 using limeand/or sodium hydroxide. After the addition of regent, the mixture wasconditioned for 10 minutes. It was filtered through a previouslyprepared 2″ thick filter bed made by DE. The DE filter bed was made byfiltering 4 gms of DE conditioned in 500 cc of water in a 4″ diameterfilter crucible. A coarse filter paper was used to form the filter bedusing a vacuum filter. The solution was filtered using a vacuumfiltration system.

Results obtained with the direct addition of different precipitatingagents are given in Table 1.

TABLE 1 Arsenic Removal from Synthetic Arsenic Solution Reagent, %Addition, gm pH Initial As, ppb Final As, ppb Removed .25 gm LaCl₃ 8.5100 26 74 .25 gm FeCl₃ 8.18 100 11 89 .25 gm AlCl₃ 8.69 100 12 88 .25 gmLa₂(CO₃)₂ 8.8 100 15 85 .25 gm La(NO₃)₃ 7.81 100 57 43 .25 gm Fe₂(SO₄)₂7.8 100 55 45 .25 gm MgCl₂ 9.11 100 42 58 .25 gm BaCl₂ 8.84 100 52 48

Results showed that arsenic can be removed by addition of chemicalagents. Removal efficiencies are between 43%-89%.

EXAMPLE B Arsenic Removal with Precipitated Mixed Hydroxides Followed byDE-Precoat Filtration Precipitated Ferric Hydroxide

0.25 gm FeCl₃ was dissolved in 100 cc of water. Ferric hydroxide wasprecipitated at pH 7.53 using lime. Different amounts of wet hydroxideswere added to a 500 cc synthetic solution of arsenic. After 10 minutesconditioning time, the solution was filtered using a DE pre-coat filter.Results are given in Table 2.

TABLE 2 As Removal with Precipitated Ferric Hydroxide Amount Added, gmpH Initial As, ppb Final As, ppb 1 8.24 100 6 .5 7.86 100 24 .25 8.11100 35

As can be seen, 1 gm of precipitated ferric hydroxide can reduce the Aslevel below 10 ppb.

Precipitated Lanthanum Hydroxide

Lanthanum hydroxide was prepared from lanthanum chloride and/orlanthanum nitrate by adding lime at pH 10.0.5 grams of lanthanum saltswere dissolved in 100 cc of water. Precipitate was collected afterfiltration. One gram of wet precipitate was added to 500 cc of arseniccontaining solutions. Results are shown in Table 3.

TABLE 3 Removal of Arsenic from Synthetic Solution using LanthanumHydroxide Amount Precipitated, gm pH Initial As, ppb Final As, ppb 17.49 100 15 1 8.12 100 32

Results showed that lanthanum hydroxide is capable of removing arsenicfrom synthetic solution.

Mixed Precipitated Hydroxides of Iron and Lanthanum for As Removal

Mixed hydroxides of iron and lanthanum were prepared by mixing FeCl₃ andLaCl₃ (prepared from lanthanum carbonate) by adding lime at pH ranges of8-9. Mixed precipitate was prepared at three different weight ratios ofFe to La. Different amounts of precipitate were added to a syntheticsolution containing 100 ppb of arsenic and mixed for 10 minutes. Resultsare shown in Table 4.

-   Schedule A: Fe:La Weight Ratio 1:1 Prepared by mixing 0.25 gm FeCl₃    plus 0.25 gm La₂(CO₃)₃ in HCl and then pH raised to 8.84 with lime-   Schedule B: Fe:La Weight Ratio 2:1 Prepared by mixing 0.25 gm FeCl₃    and 0.125 gm La₂(CO₃)₃ in HCl and then pH raised to 8.2 with lime-   Schedule C: Fe:La Weight Ratio 1:4 Prepared by mixing 0.25 gm FeCl₃    and 0.625 gm La₂(CO₃)₃ in HCl and then pH raised to 8.6 with lime

TABLE 4 Effect of Mixed Hydroxides on As Removal from Synthetic SolutionAmount of Precipitate Schedule Type Added, gm Initial As, ppb Final As,ppb A (I:1) 1 100 7 .5 100 57 .25 100 74 B (2:1) 0.5 100 25 0.25 100 45C (1:4) 0.5 100 29 0.25 100 25

Mixed hydroxide-gels of iron and lanthanum are effective to removearsenic. Different ratios of ferric and lanthanum can be mixed to removedifferent amounts of arsenic. At a ratio of 1:1, 1 gm of mixedprecipitate can effectively decrease arsenic from 100 ppb to 7 ppb.

EXAMPLE C Enhanced Coagulation of Arsenic from Fernley Drinking Waterwith Precipitate Hydroxide-gel Followed by DE Pre-coat Filtration

In these tests, drinking water obtained from a well in Fernley, Nev. wastested. The direct addition of chemical coagulant for arsenic hasseveral disadvantages. They are:

-   -   (1) needs longer conditioning to precipitate and coagulate        arsenic;    -   (2) anions such as chlorides, sulfate or nitrates are released        to drinking water which need to be removed;    -   (3) bonding of arsenate is not strong;    -   (4) pH control is required to coagulate arsenate and arsenite        ions;    -   (5) excessive reagent demand;    -   (6) size of a coagulant is small during direct addition; As a        result, incomplete removal of arsenic may occur during        separation.

In order to avoid the above mentioned problems and increase the arsenicremoval efficiency, a novel precipitated hydroxide-gel coagulant processhas been developed. This process is uniquely suitable for inexpensivepre-coat filtration using DE material. Results obtained with lanthanumhydroxides in combination with other hydroxides are given below inTables 5-7.

Tests with Pure Lanthanum Hydroxide

Five grams of lanthanum carbonate was dissolved in hydrochloric acid atpH 1.0. Lanthanum hydroxide was precipitated at pH 10.0 using lime.Precipitate was filtered. Total weight of hydroxide was 5 grams (on wetbasis). 1.25 gms of precipitate was added to Fernley drinking water. ThepH was 8.70. After different conditioning times, the water was filteredthrough a DE pre-coat filter. Results are given in Table 5.

TABLE 5 Arsenic Removal from Fernley Drinking Water Time, min. InitialAs, ppb Final As, ppb 5 68 6 10 68 2 240 68 Non-detectable

As can be seen, direct addition of precipitated lanthanum hydroxide iscapable of reducing the arsenic level in Fernley drinking water to lessthan 10 ppb in 5 minutes. After 10 minutes the arsemic level was 2 ppb.

Test with Precipitated Ferric Hydroxide

0.25 gms of FeCl₃.7H₂O was dissolved in 100 cc water. Ferric hydroxidewas precipitated at pH 7.5 using lime. Total weight of precipitate was 2grams on weight basis. One gram of wet precipitate was added to 500 ccof Fernley drinking water. The pH of the solution was 8.03. After thespecified conditioning time, the solution was filtered using DE pre-coatfiltration. Results are given in Table 6.

TABLE 6 As Removal from Fernley Drinking Water with Ferric HydroxideTime, min. Initial As, ppb Final As, ppb 5 68 15 10 68 6 240 68 5

As can be seen, precipitated ferric hydroxide is capable of removingarsenic from drinking water. After 10 minutes, the concentration of Aswas below 10 ppb.

Tests with Precipitated Lanthanum Hydroxide plus Ferric Hydroxide

In another experiment, reduced amounts of ferric chloride and lanthanumchloride were used for precipitation. In this experiment, 0.25 gmFeCl₃.7H₂O and 0.25 gm of LaCl₃ (prepared from LaCO₃) were mixed in 100cc of water. The pH was increased to 8.80 using lime. The mixedprecipitate was filtered. The total weight of precipitate was 5 grams.2.5 grams of weight precipitate was added to the Fernley drinking waterat a pH of 8.01. After conditioning for a specified time, the solutionwas filtered using DE pre-coat filtration. Results are given in Table 7.

TABLE 7 Arsenic Removal with Mixed Precipitated Hydroxides Time, min.Initial As, ppb Final As, ppb 5 68 18 10 68 7 240 68 5

Precipitated mixed hydroxide-gels of iron and lanthanum can reduce thearsenic level below 10 ppb at a lower concentration.

Effect of Mixed Hydroxide Dosage on Arsenic Removal

As described in Table 7, the mixed hydroxide precipitate was prepared bymixing an equal weight of FeCl₃.7H₂O and LaCl₃. In another series ofexperiments, mixed hydroxides were prepared by using differentiron-to-hydroxide ratios. Different amounts of mixed hydroxides wereadded to Fernley drinking water. After mixing for a specified time, thewater was filtered using a DE pre-coat filter. Results are given inTables 8-10.

TABLE 8 Tests with Ferric Hydroxide (prepared by precipitating 0.25 gmFeCl₃ with lime at pH of 7.5); total precipitate = 2.0 grams (wet);conditioning time = 10 minutes Amount of Ferric Hydroxide Gram/500 ccInitial As, ppb Final As, ppb 1 67 6 0.5 67 24 0.25 67 38

The above results showed that a preferred ferric hydroxide concentrationis 1 gm/liter (equivalent to 0.12 gm FeCl₃). Below that concentration itis difficult to reduce As from Fernley water below 10 ppb.

Effect of Direct Addition of Ferric Chloride and Ferric Sulphate

In another series of experiments, chemical grade ferric chloride andferric sulfate were directly added to Fernley drinking water. The pH ofthe water was increased to 8.2 with lime and it was mixed for 30minutes. After 30 minutes of mixing, the water was filtered through thepreviously prepared DE-bed.

TABLE 9 Tests with FeCl₃ (alone) and FeSO₄ direct addition; Fernleywater 500 cc Initial As Final As Salt, gms Concentration, ppbConcentration, ppb FeCl₃ (.25 gm) 67 11 (5 min) FeCl₃ (.25 gm) 67 <10(10 min) FeSO₄ (.25 gm) 67 21 (5 min) FeSO₄ (.25 gm) 67 <10 (10 min)

These tests show that FeCl₃ or FeSO₄ direct addition can decrease Asconcentration below 10 ppb. However, a higher amount of ferric chlorideis needed as compared to hydroxides. Further, longer mixing time and pHadjustment is required. The disadvantage of the direct addition is thatthe chloride and sulfate ions present in water have to be removed.

TABLE 10 Tests with Combined Ferric Hydroxide plus Lanthanum HydroxidePrepared by mixing 0.25 gm La₂(CO₃)₃ plus 0.25 gm FeCl₃; Total hydroxide= 5 gms (wet basis) Initial As Concentration, Final As Amount of ppt, gmppb Concentration, ppb 1 67  7 .5 67 57 .25 67-68 67-68

As can be seen, 1 gm of combined hydroxide-gel precipitate can decreaseAs concentration below 10 ppb.

TABLE 11 Tests with Combined Lanthanum Hydroxide and Ferric Hydroxide atLower Concentration; (.125 gm La₂(CO₃)₃ plus 0.25 gm FeCl₃; total ppt =1.6 gm) Initial As Concentration, Final As Amount of ppt, gm ppbConcentration, ppb .5 67 29 (5 min)  .5 67 10 (10 min) .25 67 25 (5min)  .25 67 23 (10 min)

These results show that small additions of La(OH)₃ will decrease FeCl₃addition to achieve 10 ppb.

EXAMPLE D Scale-up Test Using Mixed Hydroxides

A series of experiments were conducted using five gallons of Fernleydrinking water, in this case, hydroxide-gels prepared by mixinglanthanum chloride and ferric chloride in a ratio of 1:4. In one case,precipitation of hydroxides was conducted using chemical grade lime andin another case, chemical grade MgO was used.

Arsenic removal as a function of hydroxide-gel addition is given in FIG.2. As can be seen, hydroxide precipitate prepared using lime and MgO areeffective in removing arsenic from water. However, hydroxide-gelprepared using lime is more effective than that of the precipitateprepared using MgO.

EXAMPLE E DE-coated Hydroxides (La+Fe) Tests

In these tests, lanthanum and iron hydroxides were precipitated at aratio of 1:4 weight percent, respectively. Precipitation of hydroxideswas conducted using lime at pH 9-10. Precipitates were filtered usingthe DE-pre coat filtration method. The DE-precoat filter bed wasprepared by mixing 4 grams DE in 500 cc of water and filtering themixture through a filter device. The thickness of the bed was around 1inch thick. DE-coated hydroxide precipitate was used in the arsenicremoval test. Results are given in Table 12.

TABLE 12 Conditions: 500 cc Fernley Drinking Water; pH = 8.36 InitialAs, Conditioning Final As, % Amount, gms ppb Time, min ppb Removed 3gms, DE-coated 93 5 8 91.3 hydroxides (La + Fe) 3 gms DE-coated 93 10 693.5 hydroxides (La + Fe) 3 gms DE-coated 93 5 12 87.0 Hydroxide(Lanthanum Hydroxide only) 3 gms DE-coated 93 10 10 89.2 Hydroxide(Lanthanum Hydroxide only)

These experiments showed that DE-coated lanthanum iron hydroxides aswell as DE-coated lanthanum hydroxide are efficient in removing arsenicfrom drinking water.

EXAMPLE F Tests with Chlorinated Drinking Water Using DifferentHydroxides

Chlorinated drinking water was collected from the Fernley WaterTreatment Facility. Arsenic removal tests were conducted using differenthydroxides prepared in the laboratory. Experimental results are given inTable 13. In this case hydroxide-gels were prepared from commercialgrade lanthanum carbonate by mixing with ferric chloride andprecipitated with lime.

TABLE 13 Conditions: 500 cc of Fernley water; pH = 6.7 Amount ofhydroxide- Initial As, Conditioning Final As gels added, gm ppb time,min. Concentration, ppb % Removed 1 gm hydroxides 80 5 5 93.7 (fromlanthanum hydroxides) 1 gm (Iron + 80 10 <5 ~100 lanthanum hydroxides).25 gm 80 5 11 86.2 aluminum hydroxide .25 gm aluminum 80 10 <5 ~100hydroxide 1 gm hydroxide 80 5 11 (aluminum hydroxide + lanthanumhydroxide) 1 gm hydroxide 80 10 <5 100 (aluminum hydroxide + lanthanumhydroxide) 1 gm hydroxide ppt. 80 5 10 (aluminum hydroxide + ironhydroxide) 1 gm hydroxide ppt. 80 10 <5 100 (aluminum hydroxide)

Tests conducted with different hydroxides (prepared from theirrespective salts) were effective in removing arsenic from drinking waterusing DE-precoat filters. Arsenic removal from chlorinated water is mucheasier as compared to non-chlorinated water.

EXAMPLE G Evaluation of Different Pre-coat Filter Materials for AsRemoval

In the following tests, hydroxide-gels of iron and lanthanum were usedto coagulate arsenic from water. One gram of the hydroxide-gel sludgeprepared by combining ferric chloride and lanthanum carbonate was addedand mixed in 500 cc of Fernley water at a pH range of 8.3-8.5. Resultsare given in Table 14.

TABLE 14 Effect of Different Pre-coat Filtering Materials in As Removalafter Coagulation Amount of Type of Pre-coat Hydroxide/ ConditioningInitial As, Final As, Materials gm time, min. ppb ppb DE-bed 1 5 100 9DE-bed 1 10 100 4 Cellulose-bed 1 5 100 29 Cellulose-bed 1 10 100 26Perlite-bed 1 5 100 13 Perlite-bed 1 10 100 2 DE - Eagle-PicherDiatomaceous Earth Cellulose - Eagle-Picher, PB-40M cellulose Perlite -Eagle-Picher, CP-1400 Perlite

DE was much more effective than either perlite or cellulose. Cellulosewas not as effective as DE.

EXAMPLE H Evaluation of DE-Coated Hydroxide-Gel Mixed at DifferentRatios

In another experiment, hydroxide-gels of lanthanum and ferric werefiltered through a previously prepared DE bed. The DE containinghydroxide-gel was mixed with Fernley water for different times. Aftermixing, the water was filtered through a pre-coat DE filter bed. Resultsare given in Table 15.

TABLE 15 DE Containing Hydroxide-gel for Arsenic Removal Initial As AsConcentration in Concentration, pH of Filtrate after 5 min. ppb Solution&10 min., ppb Amount of Precipitate [DE + La(OH)₃] added to solution, gm86 3 8.45 9, 8 Amount of Precipitate [DE + La(OH)₃ + Fe(OH)₃] (La:Fe)(1:4) added to solution, gm 86 3 8.31 7, 6 Amount of Precipitate [DE +La(OH)₃ + Fe(OH)₃] (La:Fe) (1:2) added to solution, gm 86 3 8.45 5, 4

Results given in Table 15 showed that DE-containing hydroxide-gels arepowerful coagulants for removing arsenic from drinking water. It is seenthat a DE-precoat filter is uniquely suitable for DE-containinghydroxide-gels.

EXAMPLE I Preparation of DE Adsorbent

DE-coated hydroxide gels can be prepared in-situ and ex-situ. In theex-situ method, a precipitating composition is contacted with DE and thepH is adjusted to form a DE-coated hydroxide-gel. The pH may be adjustedat the same time the DE-coated hydroxide-gel is contacted with arsenic-or heavy metal-containing water. The precipitating composition ispreferably a solution of lanthanum salt and an iron salt. In the in-situmethod, metal salts are contacted with DE and the composition is aged(usually between 24-96 hours). After aging, the DE and metal salts arecontacted with arsenic- or heavy metal-containing water and the pH isadjusted to form DE-coated hydroxide-gels. As shown herein, the in-situmethod of preparation generally allows for a lower amount ofprecipitating composition to be used to provide the desired level ofarsenic or heavy metal removal.

For Examples J through L, the metal salt hydroxy-gel was coated onto DEusing two different techniques, i.e., ex-situ method and in-situ method.The hydroxide-gel was prepared by the combination of two salts(lanthanum chloride and ferric chloride) at an appropriate condition, asdescribed herein. In the in-situ preparation, DE was mixed withlanthanum chloride and ferric chloride. After 24 hours aging time, themixture was then stored for later use. The metal salt hydroxide-gelformation was conducted in-situ by mixing the prepared material withinfluent and adjusting the pH to 7.5 with NaOH. In the ex-situpreparation, DE was first mixed with lanthanum chloride and ferricchloride, and the pH was adjusted to 7.5 with the addition of NaOH sothat metal salt hydroxide-gel formation takes place. This material canbe applied directly to water.

EXAMPLE J Lab-Scale Test for Arsenic Removal

Two kinds of arsenic water samples were used in the lab test, i.e., asynthetic sample prepared with sodium arsenite and an arsenic-containinggroundwater sample obtained from Fernley, Nev. The results are shown inFIGS. 3 through 6.

The results showed that both the arsenate and arsenite species can beeffectively removed from synthetic solutions and drinking water usinghydroxy coated DE. However, the material prepared by the in-situ methodwas more effective than the material prepared by the ex-situ method. Forthe synthetic water sample, an effluent of 10 ppb or less can beachieved by applying 1000 mg/L of reagent with the ex-situ method, or500 mg/L with the in-situ method. For the Fernley groundwater sample,the required reagent dosage was 400 mg/L and 150 mg/L, respectively.

EXAMPLE K Pilot Plant Test for Arsenic Removal

Continuous flow pilot plant tests were also conducted to treat thearsenic-containing underground water. The generalized schematic of theprocess flow sheet is given in FIGS. 7 and 8. During the test, theretention time, reagent addition, pH, and the mode of reagent additionwere varied. The pilot plant was operated at a flow rate of 12-15gallons/min. Samples of influent and effluent were taken after asteady-state was reached.

Arsenic concentration in the effluent for different amounts of reagentadditions is given in FIG. 9. Approximately 100 mg/l of the reagent(prepared by the ex-situ method) is required to achieve an arsenic levelof 10 ppb. Arsenic concentration in the effluent as a function ofreagent addition prepared by the in-situ method is also shown in FIG. 9.As can be seen, reagent addition of 60 mg/l is required to accomplishthe arsenic level of 10 ppb. The results show that the in-situ method ismore efficient in adsorbing arsenic than the ex-situ method. Thisconclusion is consistent with the lab-scale test.

EXAMPLE L Heavy Metal Adsorption by Coated DE Adsorbents

Lab-scale heavy metals adsorption tests were conducted using bothin-situ and ex-situ coated DE methods. In these tests, syntheticsolutions containing Cu(II), Pb(II), and Cr(VI) were treated with coatedDE adsorbents. During the tests different contacting time was used. Theresults are shown in Table 16 and 17. Both methods showed a high removalrate for copper and lead, while in-situ method was much more effectivein treating chromium-containing water.

TABLE 16 Heavy metals removal by coated DE adsorbent (Ex-situ method)Initial Contact Time (min.) Concentration 5 min 15 min 60 min Metal(ppb) (ppb) (%) (ppb) (%) (ppb) (%) Cu (II) 9,677 59.4 99.39% 100.998.96% 141.6 98.54% Pb (II) 10,000 263 97.37% 254 97.46% 381 96.19% Cr(VI) 9,280 8260 10.99% 8250 11.10% 8270 10.88%

TABLE 17 Heavy metals removal by coated DE adsorbent (In-situ method)Initial Contact Time (min.) Concentration 5 min 15 min 60 min Metal(ppb) (ppb) (%) (ppb) (%) (ppb) (%) Cu (II) 8,064 3.8 99.95% 11.4 99.86%11.7 99.85% Pb (II) 8,333 2.8 99.97% 20.6 99.75% 24.4 99.71% Cr (VI)7,733 63.2 99.18% 46.2 99.40% 336 95.65%

All references cited herein are hereby incorporated by reference to theextent not inconsistent with the disclosure herewith. Although thisdescription contains many specificities, these should not be construedas limiting the scope of the invention, but merely providingillustrations of some of the preferred embodiments of the invention. Forexample, metal salt hydroxide gels other than those specificallyillustrated may be used. All numerical ranges given herein include allindividual values and intermediate ranges therein.

REFERENCES

-   Ahmann, D., Krumholz, L. R., Hemond, H. F., Lovley, D. R., and    Morel, F. M. M., 1997, “Microbial Mobilization of Arsenic from    Sediments of the Aberjona Watershed,” Environ. Sci. Technol., 31:    2923-2930.-   Davis, S. and Misra M., “Transport Model for the Adsorption of    Oxyanions of Selenium (IV) and Arsenic (V) from Water Onto Lanthanum    and Alumina Oxides”, Journal of Colloid & Interface Science, 188,    1997, p. 340-350.-   EPA, 2002, EPA Implements Standard of 10    ppb—http://www.epa.gov/safewater/arsenic.html Gupta, S. K. and    Chen, K. Y., “Arsenic Removal by Adsorption,” 50(3) Journal of Water    Pollution Control Federation, 493, Mar. 1978.-   Harper, T. R. and Kingham, N. W. “Removal of Arsenic from Wastewater    using Chemical Precipitation Methods,” 64(3) Water Environment    Research 200-203, 1992.-   Merrill, D. T., et al., “Field Evaluation of Arsenic and Selenium    Removal by Iron Coprecipitation,” 6(2) Environmental Progress 82-90,    1987.-   Misra, M. and Nayak, D., U.S. Pat. No. 5,603,838.-   Misra, M., et al., U.S. Pat. No. 6,197,201.-   Misra, M., Raichur, A. M. and Keltner, K., “Adsorption and    Separation of Arsenic from Process Water Using LS™ (Lanthanum-Silica    Compound),” Proceedings of the Randol Gold Forum '97, 1997, pp.    203-206.-   Misra, M. and Adutwum, K. O., “Adsorption of Oxyanions of Selenium    Onto Lanthanum Oxide and Alumina,” Minor Elements 2000, Published by    SME, February 2000, pp. 345-353.-   Misra, M., Nanor, J., and Bucknam, C. H., “Enhanced Precipitation    and Stabilization of Arsenic from Gold Cyanidation Process,” Minor    Elements 2000, Published by SME, February 2000, pp. 141-148.-   Nanor, J. and Misra, M., “Removal and Stabilization of Arsenic,”    Randol Gold Forum, 1999, pg. 191-196.-   National Academy of Sciences, “Arsenic-Medical and Biological    Effects of Environmental Pollutants,” U.S. Government Printing    Office, Washington, D.C., 1977. National Research Council, Arsenic    in Drinking Water, Washington D.C., National Academy Press, 1999.-   Rawat, A. and Misra, M., “Adsorption of the Oxyanions of Arsenic    onto Lanthanum Oxide,” EPD Congress, The Minerals, Metals and    Materials Society (TMS), Warrendale, Pa., 1998, pp. 14-23.-   Wasay, S. A., et al. “Removal of Arsenite and Arsenate Ions from    Aqueous Solution by Basic Yttrium Carbonate,” 30(5) Wat. Res.    1143-1148, 1996.

1. A method of removing arsenic from arsenic-containing watercomprising: contacting said water with a precipitating compositioncomprising a metal salt hydroxide-gel.
 2. The method of claim 1, furthercomprising separating said water from said precipitating composition. 3.The method of claim 2, wherein said separating is performed using afilter.
 4. The method of claim 3, wherein said filter is a diatomaceousearth filter bed coated with a metal salt hydroxide-gel.
 5. The methodof claim 1, wherein said metal salt hydroxide-gel is formed prior tocontacting said water with said precipitating composition.
 6. The methodof claim 1, wherein the pH of the precipitating composition andarsenic-containing water is adjusted to form a metal salt hydroxide-gelafter contacting the precipitating composition and said water.
 7. Themethod of claim 3, wherein said filter is selected from the groupconsisting of: diatomaceous earth, cellulose, and perlite.
 8. A methodof removing heavy metals from heavy metal-containing solutioncomprising: contacting said solution with a metal salt hydroxide-gel;and passing said solution through a filter.
 9. The method of claim 8,wherein said metal salt hydroxide-gel comprises lanthanum and iron. 10.The method of claim 8, wherein said filter is a diatomaceous earthfilter bed.
 11. The method of claim 8, wherein said filter is adiatomaceous earth filter bed coated with a metal salt hydroxide-gel.12. A composition for removing arsenic or heavy metal from arsenic- orheavy metal-containing water comprising: diatomaceous earth, a lanthanumsalt and an iron salt.
 13. The composition of claim 12, wherein saidlanthanum salt and iron salt are chlorides.
 14. The composition of claim12, wherein said metal is arsenic.
 15. The composition of claim 12,wherein said metal is a heavy metal.
 16. A method of using thecomposition of claim 12, comprising mixing the precipitating compositionwith arsenic- or heavy metal-containing water and adjusting the pH sothat a metal salt hydroxide-gel is formed.
 17. A method of making adiatomaceous earth-coated hydroxide-gel comprising: contacting aprecipitating composition with diatomaceous earth; and adjusting the pHto form a diatomaceous earth-coated hydroxide-gel.
 18. A method ofmaking a diatomaceous earth-coated hydroxide-gel comprising: contactingmetal salts with diatomaceous earth, forming a composition; and agingthe composition.
 19. The method of claim 18, further comprisingseparating the solid composition from the liquid.
 20. The method ofclaim 18, further comprising contacting arsenic-containing water withthe composition; and adjusting the pH of the resulting composition toform a diatomaceous-coated gel.